1. Field of the Invention
The present invention relates generally to an ultrasound imaging system, and, more particularly, to a method for the optimal design of an apodization function used for non-uniform weighted periodic sparse arrays for an ultrasound imaging system.
2. Description of the Related Art
An ultrasound imaging system converts electric signals into ultrasonic waves using an ultrasound transducer (for example, a piezoelectric transducer), sends the ultrasonic waves, receives ultrasonic signals reflected from a target object, converts the ultrasonic signals into electric signals, signal-processes the electric signals into images, and transmits the resulting images to a user. A Brightness (B)-mode image, which is a basic ultrasound image, represents clinical information about a section of a human body. The quality of the B-mode image depends on performance such as resolution, signal-to-noise ratio and frame rate.
In a current ultrasound system in which an array transducer composed of array elements is used as an ultrasound transducer, the distance between the array elements is determined based on the wavelength of a transmission signal and then the number of array elements used, that is, the number of channels, is determined to be suitable for the size of a desired aperture. Meanwhile, focusing on desired image points is achieved by applying electrical variable time delay to each channel.
In the present specification, a typical array using all array elements within the size of an aperture is referred to as a Fully Sampled Array (hereinafter referred to as an ‘FSA’). A typical ultrasound system using FSAs is configured to increase the size of an aperture, that is, the number of channels, so as to obtain higher resolution and employs a high performance focusing technique such as dynamic focusing. Here, since each channel includes an ultrasound transmission/reception circuit and a time delay calculator for focusing, the complexity of the hardware of an ultrasound system increases in proportion to the number of channels. A one-dimensional array transducer for providing second-dimensional sectional images, which is widely used, currently has 64˜256 channels, the number of which is gradually increasing. In the case where a two-dimensional array transducer for providing three-dimensional images is composed of 64 channels in the lateral direction and 64 channels in the elevation direction, a total of 4096 channels are required, so that the size of hardware is increased 64 times that of the one-dimensional array transducer that uses 64 channels.
In order to solve this problem, various techniques for reducing the complexity of hardware have been proposed. Among these techniques, a sparse array technique can obtain the effect of seeming to use a large aperture using a limited number of channels, and thus it is one of the principal search issues.
A sparse array technique is a technique for reducing the number of channels while minimizing the reduction in lateral resolution by sparsely distributing the locations of array elements within an aperture required by an image using only some of the array elements. The sparse array technique is used as a technique for effectively increasing the size of an aperture for a given number of channels. Since the sparse array technique uses typical dynamic reception focusing, it has advantages in that there is no motion artifact and additional hardware is not required, unlike the extended aperture technique and the combined aperture technique. However, in the sparse array technique, the distance between array elements actually used is increasing, and thus unwanted grating lobes are generated. Accordingly, the most important aspect to be considered in the design of a sparse array is finding a method of preventing the occurrence of grating lobes or performing their suppression so as to allow the generation of only a minimum of grating lobes under given design conditions.
Among such sparse arrays, a periodic sparse array is configured to regularly distribute array elements within an aperture. A periodic sparse array has advantages in that the design thereof is very simple and the number of channels can be reduced in proportion to the period of the distribution of array elements, but has a disadvantage in that grating lobes are generated because the distance between the array elements increases in proportion to the period. Various techniques for eliminating the grating lobes of a periodic sparse array have been proposed.
Meanwhile, in an ultrasound system using typical array transducers, that is, FSAs, the level of side lobes is suppressed by applying a weight function to an aperture function, as shown in Equation 1:
                                          Ψ                          N              ch                                ⁡                      (                          u              ′                        )                          =                              ∑                          n              =              0                                                      N                ch                            -              1                                ⁢                                                    w                ⁡                                  (                  n                  )                                            ·                                                a                  0                                ⁡                                  (                  n                  )                                                      ⁢                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                      d                      ⁢                                                                                          ⁢                                                                        u                          ′                                                λ                                                              )                                                  ⁢                n                                                                        (        1        )            where w(n) is an apodization function that is applied to an aperture function, a0(n) is an aperture function that represents the weight of an n-th array element, and Nch is the number of channels.
Widely-used apodization functions include a hanning window function and a hamming window function. FIG. 1(a) shows a unidirectional beam pattern for a uniform weighted FSA that does not use an apodization function, and FIG. 1(b) shows a unidirectional beam pattern for a non-uniform weighted FSA that uses an apodization function as a hanning window function. From FIG. 1, it can be seen that in the case where an apodization function is used, the width of a main lobe is somewhat increased and the levels of side lobes are rapidly suppressed.
Accordingly, it can be easily expected that the application of a weight function to a uniform weighted periodic sparse array can improve the Destructive Beam Cross-interference (DBC) effect. Accordingly, in the present invention, a periodic sparse array to which an apodization function is applied is referred to as a ‘non-uniform weighted periodic sparse’. However, the non-uniform weighted periodic sparse array using a typical apodization function has problems in that the width of the main lobe thereof is greater than that of the uniform weighted periodic sparse array, the Signal-to-Noise Ratio (SNR) thereof is degraded because the amplitude of a signal is decreased after the focusing of a beam, and the complexity of the system is increased because the transmission and reception circuit thereof requires additional hardware.
In order to resolve the above problems, various methods of designing a non-uniform weighted sparse array have been proposed. The most popularized method of designing a non-uniform weighted sparse array includes a method using an optimization algorithm, such as a genetic algorithm, so as to optimize a final beam pattern. This approach has an advantage in that an optimal apodization function can be designed for a certain sparse array and it is difficult to implement actual hardware because an apodization function has a complex function form.
Furthermore, another non-uniform weighted sparse array design method for suppressing grating lobes is a method of regarding grating lobes as noise and designing the apodization function of a sparse array based on the concept of a filter for eliminating the noise. This case has an advantage in that an apodization function can be configured in a simple structure because designing is performed using a signal processing technique. However, in the case where an apodization function is designed based on such a concept of a filter, the orders of a filter should be limited because the size of the aperture of an array transducer, that is, the number of array elements, is limited. Accordingly, there may occur the cases where an apodization function designed based on the concept of a filter does not acquire desired performance for a given sparse array.
Meanwhile, a non-uniform weighted sparse array may be designed using the effective aperture concept, which is a design method for a Vernier array. The method using the effective aperture concept is a method of designing an apodization function so that an effective aperture obtained in the case where the apodization function is applied to a given sparse array is similar to the effective aperture of an FSA, compared to that in the contrary case. Since in this method, an effective aperture is defined as the convolution of a transmission/reception aperture function, the effective aperture of a given sparse array can be optimized using one of various signal processing techniques. However, since this method is not an analytic approach, like the design method for a uniform weighted periodic sparse array, it is impossible to find the conditions of an optimal apodization function for a certain periodic sparse array from the point of view of the cancellation of grating lobes.
Accordingly, in the present invention, there is proposed a method for the optimal design of a non-uniform weighted periodic sparse array that enables the design of a weight function capable of optimizing a final beam pattern by effectively suppressing other excessive grating lobes than common grating lobes for a periodic sparse array.